The aircraft involved in the Northwest Seaplanes crash occurring off Whidbey Island on September 4, 2022, was a de Havilland DHC-3 Otter registered as N725TH.
The Otter is a large, single-engine aircraft popular for its ability to fly in and out of unimproved areas and operate from water when equipped with floats. Produced between 1951 and 1967, more than 400 were ultimately produced.
The original aircraft were powered by 600 horsepower Pratt & Whitney radial engines. Some aircraft, including N725TH, were extensively modified by replacing their heavy and complicated radial engines with lighter and more powerful turbine engines. This modification was intended to increase both efficiency and reliability.
At this point, no official determination has been made regarding the cause of the Northwest Seaplane crash. However, it is known that the aircraft suddenly departed a stable cruise at approximately 800 feet above the water, crashing into the water in a matter of seconds.
One potential cause of such a sudden, uncontrolled, descent is a catastrophic structural or mechanical failure. Especially with older aircraft, a review of historical incident and accident data can shed light on problem areas for a given aircraft design.
There are 37 incidents and accidents listed in the NTSB accident database involving the de Havilland DHC-3 Otter.
Of those thirty-seven events, five involved structural or mechanical failures that impacted the flight control system of the aircraft.
A search of accidents and incidents recorded on the Transportation Safety Board of Canada (TSB) website yielded fifteen more events involving the DHC-3. Of those fifteen events, three involved uncontrolled descents.
One of these crashes occurred on a steep mountainous slope that presented a significant avalanche risk to investigators. Accordingly, no conclusive determination regarding the cause was able to be made, however, weather was suspected to be a significant contributing factor. The other two crashes both involved catastrophic structural failures.
To better understand the causes of these events, a review of the DHC-3’s pitch control system should be helpful.
The DHC-3 is equipped with a conventional cruciform tail where pitch is controlled by moveable elevators connected to an adjustable horizontal stabilizer. The elevators are connected to the pilot’s control column using cables and are deflected up or down by moving the column forward or back. The aircraft can be trimmed to maintain a desired attitude by adjusting the angle of the horizontal stabilizer. This system uses cables connecting a hand wheel in the cockpit to a jackscrew in the tail connected to the stabilizer. The trailing edge of both elevators are equipped with movable trim servos. These smaller control surfaces are a passive system designed to reduce the input force necessary for the pilot to move the elevator aerodynamically, functioning a bit like power steering.
The Canadian Incidents/Accidents
● March 31, 2011, Otter C-GMCW, experienced a catastrophic in-flight break-up. No specific determination was able to be made regarding the ultimate cause, but the TSBC did determine that the break-up was the result of an overspeed condition after the aircraft diverged from straight and level flight for an unknown reason.
● On May 9, 1996, the right wing of DHC-3 C-GBTU, separated from the aircraft while on approach to a water landing. In this case, the TSB was able to determine that the Otter’s right wing lift strut had failed due to a fatigue crack that had developed through one of the strut’s mounting bolt holes. Both Transport Canada and the FAA have issued Airworthiness Directives mandating heightened inspection schedule of the Otter’s lift struts. Fatigue cracking can be rooted in design deficiency, or in the hard use planes like these see. Competent and thorough inspection regimes are particularly important for Otters due to the advancing age of the airframes in the fleet.
The U.S. Incidents/Accidents
● On June 20, 1989, DHC-3 Otter N41755 unexpectedly pitched nose down when the horizontal stabilizer jack-screw failed in flight. The cause was determined to be improper lubrication and excessive wear.
This type of failure is the result of poor maintenance and inspection practices. Notably, this event was very similar to the cause of the Alaska Airlines Flight 261 crash on January 31, 2000, where the stabilizer jackscrew also failed.
● On February 4, 1992, DHC-3 Otter N13GA unexpectedly pitched nose-down in cruise flight when the aircraft’s left elevator trim servo aft rivets tore out. This incident, when compared with the event above, is a good example of how failures of different systems can result in the same effect. As shown below, failures of the trim servo have occurred multiple times in the Otter and indicate a design issue, even though poor maintenance and inspection practices also contributed to the failures.
● On July 13, 1995, DHC-3 Otter N472PM experienced a dramatic vibration after take-off. Inspection upon landing indicated that the elevator trim servo had cracked and partially separated.
● On December 28, 2002, DHC-3 Otter N3904 experienced a loss of rudder control after one elevator partially came off its hinges and blocked the movement of the rudder. The elevator torque tube that connects both elevators showed signs of being replaced improperly. Again, this looks more like poor maintenance and inspection than being the result of deficient aircraft design.
● On May 30, 2014, DHC-3 Otter N3125N unexpectedly pitched nose-down in cruise flight when the aircraft’s left elevator trim tab separated. On this particular aircraft, the trim tab was modified by the current DCH-3 type certificate holder, Viking Air Limited, ostensibly to prevent flutter – a catastrophic condition where flight surfaces vibrate, potentially to the point of failure. In this instance, the FAA recognized that a safety issue was presented by the installation of the more powerful turbine engines in the Otter. Apparently, the prop wash from the turbine engine’s propeller resulted in increased airflow over the control surfaces beyond the original design parameters. This increase in airflow had resulted in trim servo flutter and ultimately servo failure.
Airworthiness Directive (AD 2004–05–01) was issued to address modified trim tabs installed on turbine engine equipped Otters.
Due to continuing flutter issues in turbine Otters, another Airworthiness Directive (AD 2011–12–02) was issued to reduce the maximum airspeed allowed for these aircraft (134 mph when equipped with floats).
The fact that the Type Certificate holder modified the original design of the trim servo, and that the modification was also deemed to be inadequate for the original design envelope, illustrates a significant weakness in the Otter design.
Moving forward, ongoing problems with the Otter trim servo system, especially in turbine-equipped Otters, is an area NTSB investigators will likely focus on as they try to determine the cause of the Northwest Seaplanes crash.
One other possibility that cannot be ruled out at this early stage, and which might result in an emergency flight path similar to that of N725TH, is if its turbine engine had reverse thrust capability (via propeller blade pitch change), and it somehow flipped into reverse thrust during cruise flight, that would instantly counteract forward flight (like pushing the plane backwards), and could cause it fall from the sky. While there are typically numerous safeguards designed into the reverse thrust systems to prevent use in flight, some systems have failed.
*Kerry Kovarik is an aviation accident attorney at Aviation Law Group PS in Seattle. He is also a commercial pilot, flight instructor, and FAA certified airframe and powerplant mechanic with inspection authority. Kerry represents victims and families in aviation accident cases.